1. Field of the Invention
The present invention relates to a method of injection-compression molding using an injection-compression molding machine.
2. Description of the Related Art
A method of injection-compression molding using a conventional injection-compression molding machine comprises an injection process of placing a fixed mold and a movable mold (hereinafter referred to as a "mold") in a closed state at an injection mold opened position which corresponds to a state opened by a predetermined mold opening stroke relative to a final mold clamping position to be taken when a compression process is completed, and injecting molten resin to fill in a cavity of the mold placed in the closed state at the injection mold opened position; and a compression process for compressing the mold, following the injection process, by pressing the mold in the closed state until the mold reaches the final mold clamping position.
However, according to the above conventional technique, in case an injection-compression molding machine provided with a toggle type mold clamping apparatus is used, when molten resin is injected to fill in the cavity of the mold placed in the closed state at the injection mold opened position, a tie bar of the machine is elongated due to the injection pressure of the molten resin concurrently causing a link shrinkage, and hence the mold opening stroke is increased more than that of the injection mold opened position by a stroke corresponding to the sum of the tie bar elongation and the link shrinkage. Further, with an injection-compression molding machine provided with a straight hydraulic mold clamping apparatus, the tie bar is elongated and a clamping ram is concurrently contracted, and hence the mold opening stroke is increased more than that of the injection mold opened position by a stroke corresponding to the sum of the elongation of the tie bar and the contraction of the clamping ram. When the mold opening stroke is increased in the injection process in this way, a minute relative slip is generated between the mold and a skin layer which is produced when the injection-filled molten resin is cooled by the mold, resultantly deteriorating the transferability which is a merit of the injection-compression molding method. More particularly, the deterioration of the transferability is undesirably when producing a mold such as a Fresnel lens or a light guide plate which has minute surface gradation.